Hello, everyone!
Today, I would like to share with you an article about Passive Optical Network.
PON is the technology most commonly used to deliver fiber-based broadband to homes and businesses. PON adopts a point-to-multipoint design, which means the network is structured as a tree, with a single point starting in the ISP’s network and then fanning out to reach up to 1024 homes. PON gets its name from the fact that the splitters are passive: they forward optical signals downstream and upstream without actively storing-and-forwarding frames. In this way, they are the optical variant of repeaters used in the classic Ethernet. Framing then happens at the source in the ISP’s premises, in a device called an Optical Line Terminal (OLT), and at the end-points in individual homes, in a device called an Optical Network Unit (ONU).
The following figure shows an example PON, simplified to depict just one ONU and one OLT. In practice, a Central Office would include multiple OLTs connecting to thousands of customer homes. For completeness, Figure 52 also includes two other details about how the PON is connected to the ISP’s backbone (and hence, to the rest of the Internet). The Agg Switch aggregates traffic from a set of OLTs, and the BNG (Broadband Network Gateway) is a piece of Telco equipment that, among many other things, meters Internet traffic for the sake of billing. As its name implies, the BNG is effectively the gateway between the access network (everything to the left of the BNG) and the Internet (everything to the right of the BNG).
An example PON that connects OLTs in the Central Office to ONUs in homes and businesses.
Because the splitters are passive, PON has to implement some form of multi-access protocol. The approach it adopts can be summarized as follows. First, upstream and downstream traffic are transmitted on two different optical wavelengths, so they are completely independent of each other. Downstream traffic starts at the OLT and the signal is propagated down every link in the PON. As a consequence, every frame reaches every ONU. This device then looks at a unique identifier in the individual frames sent over the wavelength, and either keeps the frame (if the identifier is for it) or drops it (if not). Encryption is used to keep ONUs from eavesdropping on their neighbors’ traffic.
Upstream traffic is then time-division multiplexed on the upstream wavelength, with each ONU periodically getting a turn to transmit. Because the ONUs are distributed over a fairly wide area (measured in kilometers) and at different distances from the OLT, it is not practical for them to transmit based on synchronized clocks, as in SONET. Instead, the OLT transmits grants to the individual ONUs, giving them a time interval during which they can transmit. In other words, the single OLT is responsible for centrally implementing the round-robin sharing of the shared PON. This includes the possibility that the OLT can grant each ONU a different share of time, effectively implementing different levels of service.
PON is similar to Ethernet in the sense that it defines a sharing algorithm that has evolved over time to accommodate higher and higher bandwidths. G-PON (Gigabit-PON) is the most widely deployed today, supporting a bandwidth of 2.25-Gbps. XGS-PON (10 Gigabit-PON) is just now starting to be deployed.
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